1. Field of the Invention
The present invention relates to a non-magnetic mono-component color toner, and more specifically to a non-magnetic mono-component color toner having a narrow charge distribution, good chargeability, good environmental independence, superior image characteristics, high transfer efficiency, and excellent long-term stability and image quality.
2. Description of the Related Art
The recent hard-copying and printing techniques using image formation methods such as electrophotographs, are rapidly moving toward full color from black and white. In particular, the color printer market is expanding very rapidly. In general, formation of color images by full color electrophotography is carried out with three colors comprising cyan, magenta, and yellow, or four colors further comprising black, to reproduce all colors. In this rapidly growing full color market, high image quality, good reliability, compactness, light-weight, low price, high speed, and furthermore environmental aspects such as low energy consumption, recyclability and so forth are strongly required. Improvement and development of image formation methods and toners to satisfy these requirements are widely in progress.
In general, image formation in electrophotography includes the steps of uniformly charging a drum surface, exposing the drum surface and forming an electrostatic latent image, developing the latent image on the drum surface using a toner formed on the surface of a developing roller and obtaining a toner image, transferring the toner image, fixing the toner image, and, a cleaning step that removes the toner remaining on the drum surface from the transfer step.
Each step of the image formation process in electrophotography requires the following basic characteristics from a toner. The developing step requires an appropriate charging of the toner, charge maintenance, and environmental independence. The transfer step requires good transfer characteristics. The settlement step requires the characteristics of low-temperature fixing and offset resistance. And lastly, the cleaning step requires good cleaning performance and contamination resistance. Recently, the above characteristics have become more important with the trend toward high resolution, high speed, and full color.
With regard to long-term maintenance of image quality for repeated printing, there is a method of mixing four colors directly in a photoconductive drum in the transfer step. And recently, indirect transfer image formation has been mainly used in full color printers because it can offer high speed and good image quality. In indirect transfer image formation, a toner image of each color on the drum surface is repeatedly transferred to an intermediate transfer belt, and then the image as a whole is transferred onto paper.
However, because indirect transfer image formation requires more toner transfer steps, charging elements can be more easily contaminated, thereby making it difficult to obtaining accurate transfer performance.
In order to obtain stable long-term and high-quality full color images, research on additives, toner shape, surface structure control, and so forth are required to obtain higher charging stability and to minimize the contamination in charging elements.
With regard to the cleaning step, reduction of remaining toner after transfer and reducing the cleaner size are important tasks for improving environmental independence.
To overcome these problems of the transfer and cleaning steps, it is important to maintain uniform electrification. One of the characteristics required for this purpose is to prevent the deterioration of electrification which is caused by contamination in charging elements. For this purpose, it is best not to use material which contaminates the charging elements, but when such material is essential, an additive can be used that makes cleaning easier by decreasing the adhesive force of the charging elements. Although the former situation is ideal, in actual toner manufacturing processes, it is not always possible to use materials with desired characteristics, so the latter is mostly the case. In addition, it is important to reduce the remaining toner amount, and thus to improve and maintain transfer efficiency of the toner.
Fine particles such as silica may be added to the toner to reduce its adhesiveness to the photoconductive drum. However, in case of using fine silica particles, it is intervened between toner and drum to reduce the toner's adhesiveness to the drum and improve its transfer efficiency, so in order to obtain good transfer efficiency, the level of cover of the toner surface by the particles has to be high. Consequently, the amount of added fine particles increases and the toner charging characteristics become poor. Moreover, the fine particles may adhere to electrostatic latent image carriers, and filming or fixing problems may occur. In particular, silica particles may cause problems of image density irregularity at low temperature and humidity, and non-image area contamination at high temperature and humidity, because they are highly environment-dependent.
Korean Patent Publication No. 1996-0024716 discloses a toner for electrophotography comprising a fine powder fatty acid metal salt the surface of which is treated with hydrophobic silica, or fine powder of a melted mixture of fatty acid metal salt the surface of which is treated with hydrophobic silica, and resin. In addition, Korean Patent Publication No. 2003-0056152 discloses the mono-component magnetic toner composition comprising a mother particle of magnetic toner, hydrophobic silica having specific surface area of 20-80 m2/g, hydrophobic silica having specific surface area of 130-230 m2/g, and metal oxide fine powder. However, the documents do not disclose the spherical organic fine powder which is an essential component of the present invention.
As a method for improving environmental independence of a toner, addition of inorganic fine particles having electric resistance lower than that of silica particles and good electrification, such as titanium oxide particles, is known. However, if inorganic fine particles having lower electric resistance are used, charge distribution of the toner may change easily. Also, it may result in poor second transfer when using an intermediate transfer belt or retransfer of wrong sign color toner during multiple transfers.
A method of increasing resistance of inorganic fine particles by treating the surface with a silane coupling agent, etc. was proposed to solve this problem. However, cohesion of the fine particles becomes so severe that their dispersibility on the toner surface decreases. Also, fluidity of the toner may decrease or blocking may occur due to free cohered particles.
Accordingly, research on a color toner having a narrow charge distribution, good charging characteristics, environmental independence, and long-term stability without contamination of the charging elements, is in much demand.